Valves have long been used to control the volume of flow of fluids or gas in a piping system, for example, heating, ventilation or air-conditioning system. Conventional valves may include on/off control valves and percentage control valves. An on/off-type valve usually includes a valve stem that moves through its entire length between a fully-open and a fully-closed state or position. Unlike on/off control valves, the valve stem of a percentage control valve primarily actuates through a percentage of the available stem stroke length. Although percentage control valves may perform precise control of the volume of flow in a piping system, they are more expensive than on/off control valves.
A conventional butterfly valve is usually employed as an on/off control valve, and it has also been used as a percentage control valve in a piping system to achieve the cost efficiency of an on/off control valve and the ability to precisely control flow volume of a percentage control valve. The characteristics curve of a butterfly valve when used for percentage control is shown in FIG. 1. Referring to FIG. 1, the horizontal coordinate is the stem stroke of a valve stem of a butterfly valve, and the vertical coordinate is the flow ratio between the current volume of flow and the maximum volume of flow of a butterfly valve. Straight line L represents an ideal relationship between stem stroke and flow ratio in percentage control pf flow. Curve S represents an actual situation of a butterfly valve used for percentage control.
Curve S may be divided into three regions A, B and C. Region A corresponds to approximately the 0-th to 28th percentage of the entire stem stroke, and is only responsible for the first 10% of the maximum volume of flow. Therefore, a butterfly valve operates with inefficiency in region A. Likewise, region C corresponds to approximately the 75th to 100th percentage of the entire stem stroke, and is only responsible for the last 10% of the maximum volume of flow. Therefore, region C is also an inefficient operation region for the butterfly valve. In addition, the valve stem of a butterfly valve may be susceptible to oscillation when the butterfly valve operates in region A or C.
Region B corresponds to approximately the 28th to 75th percentage of the entire stem stroke, and is responsible for 80% of the maximum volume of flow. Unlike region A or C, region B is an efficient operation region for a butterfly valve. However, as compared with ideal line L, curve S prior to the intersection point W represents that the butterfly valve provides insufficient volume of flow, and represents excessive volume of flow after the intersection point W. This is known as a “hunting problem.” Specifically, it is difficult for a butterfly valve to “hunt” for a desired output level when it is operating in region B.